1. Field of the Invention
The present invention relates to packet transfer apparatuses, and more specifically, to a packet transfer apparatus which terminates Layer 2 Tunneling Protocol (L2TP) on a subscriber side.
2. Description of the Related Art
One known system uses Point-to-Point Protocol (PPP) to authenticate a subscriber as a user when a subscriber terminal is connected via an Internet service provider (ISP) to the Internet. PPP is a protocol for making a one-to-one connection between a subscriber terminal and an ISP, at a level equivalent to layer 2. PPP was originally used in an environment where a terminal is connected directly to an ISP access point by phone line, authenticated, and then connected to the Internet. As continuous access to the Internet has become common, the connection between the subscriber terminal and the ISP server is currently made through an access carrier network utilizing Internet Protocol (IP), besides the telephone network. Because the access carrier network is formed in layer 3, a means for transferring a PPP packet to the PPP terminating apparatus of the ISP is required to perform PPP authentication through the access carrier network. L2TP can be used as the transfer means.
L2TP allows a logical private line to be formed between a subscriber terminal and an ISP in the access carrier network and allows PPP to be terminated at an exit from the access carrier network (to the ISP side). The logical private line is referred to as an L2TP tunnel and an L2TP session. An L2TP connection is made by an L2TP terminating apparatus (L2TP access concentrator (LAC)) on the subscriber side, and an L2TP terminating apparatus (L2TP network server (LNS)) on the ISP side. A PPP packet is transferred by the L2TP connection and terminated by the LNS. Because the LNS terminates both L2TP and PPP on the ISP side, the number of L2TP sessions increases with an increase in the number of subscribers. PPP connection extension by L2TP and the functions of the LAC and LNS are described in Request for Comment (RFC) 2661.
Japanese Unexamined Patent Application Publication No. 2002-164930 discloses a communication method utilizing an IP tunneling technology such as L2TP for connecting an ISP and a multicast protocol or a multicast routing protocol such as Internet Group Management Protocol (IGMP) incorporated in a general router or layer-3 switch, for distributing multicast contents.
In an environment in which an L2TP connection is made in the access carrier network to connect a subscriber and an ISP, there is generally a high demand for reducing traffic between the LAC and the LNS, in order to reduce the cost of the access carrier network.
In a system where a server on the Internet multicasts a program and the subscribers receive multicast packets, traffic between the LAC and the LNS would be reduced by copying the multicast packets in the LAC and distributing the copied packets to the subscribers. In an access network of the L2TP type, a user IP packet is transferred by a PPP connection, but a general LAC just sends and receives the PPP connection and does not perform processing on the IP packet level. The LAC cannot manage multicast destinations. According to an IETF draft (draft-ietf-12tpext-mcast-03.txt), the LNS manages and gives a list of multicast destinations to the LAC, so that the LAC can distribute copied multicast packets. Each time a request is made to add or delete a subscriber to or from the multicast packet group in this system, the LNS must update the destination list and send the updated list to the LAC. This sequence causes a delay in actual addition or deletion of the subscriber and wastes a frequency band for transferring the destination list.